Magnetic field-enhanced plasma etch reactors for the plasma etching of a substrate are well known in the art. The use of plasma-based reactor processes has become widespread throughout the semiconductor industry, due in part to the ability of these devices to provide precisely controlled thin-film depositions and etches.
A plasma reactor typically comprises a reaction chamber supplied with a reactant gas, a pair of spaced-apart electrodes (a cathode and an anode) that utilize radio frequency (Rf) energy to generate an electric field within the chamber and thereby ionize the gas, and a substrate support which is adapted to support a substrate within the electric field. The cathode is typically embedded in the substrate support, and the anode is typically positioned over the substrate.
When the electric field ionizes the reactant gas, a plasma is produced which is a mixture of electrons, cations and anions. In operation, the positively charged cations are pushed outward by mutual repulsion toward the surfaces of the reaction chamber and are electrically attracted toward the cathode. As a result the substrate, which is supported just above the cathode, is bombarded by positively charged ions. Depending upon the composition of the plasma, this can result in either the formation of a deposited layer on the substrate, or can result in the substrate material being etched.
A magnetic field is generated within the reaction chamber to control the motion of the electrons within the plasma, and to restrict free electrons to a bulk plasma region located centrally in the reaction chamber. Typically, the magnetic field is generated through the use of one or more electromagnets that are positioned about the circumference of the reaction chamber. The magnetic field so generated is perpendicular to the electric field within the chamber and parallel to the surface of the cathode. In particular, the magnetic field is oriented perpendicular to the electric field within a cathode plasma sheath proximate to the surface of the cathode. This orientation of the magnetic field confines a substantial quantity of electrons to a region within the plasma around the cathode plasma sheath, thereby increasing the plasma density near the cathode sheath and the substrate. Absent corrective measures, an uneven distribution of plasma density results due to a phenomenon referred to as E×B drift. Since the plasma is not uniform in density, etch rates experienced by a substrate exposed to the plasma are found to vary across the substrate, such that regions exposed to the greatest plasma density experience the highest etch rates. E×B drift, and the factors giving rise to it, are discussed in greater detail in U.S. Pat. No. 6,113,731 (Shan et al.), and the directional effect of this phenomenon is illustrated with respect to FIGS. 1A and 1B of that reference.
In addition to causing variations in etch rates across the surface of a substrate, E×B drift also results in the spatially non-uniform accumulation of electrical charge on the substrate. The differential in electrical charge produces voltages and current flow between different points on the wafer, a condition referred to as “charge up”. If the voltage across any of the dielectric structures fabricated on the wafer exceeds the maximum safe voltage of that structure, then the structure may be damaged by the current flowing through it.
Various techniques have evolved in the art in an attempt to compensate for E×B drift and/or to avoid charge-up. The goal of many of these techniques is to promote greater uniformity in plasma distribution and more uniform etching of a substrate. One approach has been to employ a rotating magnetic field to more uniformly distribute plasma density differences. The rotating magnetic field is generated using two pairs of electromagnetic coils located on opposite sides of the reaction chamber. The coils in each coil pair are coaxially aligned with one another. Current flowing in one pair of coils produces a magnetic field perpendicular to the magnetic field produced by a current flowing in the other pair of coils, i.e., the current flows in opposite directions through adjacent coils. When the pairs of coils are driven by sinusoidal currents that are 90° out of phase, the coils create a rotating magnetic field parallel to the upper surface of the substrate in the reaction chamber.
While the use of a rotating magnetic field relative to the wafer can significantly reduce time-average spatial non-uniformity in the ion flux bombarding the wafer and thus can provide greater uniformity in etch rates, as noted in U.S. Pat. No. 6,113,731 (Shan et al.), it does not improve the instantaneous spatial uniformity of the ion flux on the wafer surface. Consequently, this approach does not avoid charge up damage to the components of a wafer substrate.
Shan et al. addresses this problem through the use of a magnetic field pattern in which the direction of the magnetic field at any point within the reaction region is approximately the vector cross-product of (1) the gradient of the magnitude of the magnetic field at that point, and (2) a vector extending perpendicularly from the substrate surface toward the plasma. The direction of this magnetic field produces an E×B drift of electrons in the pre-sheath away from the region where the magnitude of the magnetic field is highest, and toward the region where the magnitude is lowest. Consequently, the resulting E×B drift counteracts to some extent the tendency for the rate of production of free electrons to be highest in the region where the magnitude of the magnetic field is highest.
While the approach of Shan et al. represents a notable improvement in the art and does improve the instantaneous spatial uniformity of the ion flux on the wafer surface, it has been found that, even with the use of this approach, some non-uniformities in plasma densities and etch rates still persist, especially at regions of the substrate disposed closest to the vertices formed by adjacent electromagnets. This type of phenomenon, which is known as a “plasma depletion effect”, can still result in substrate damage and non-uniform etch patterns in the regions in which it occurs.
There is thus a need in the art for a method for achieving greater uniformity in plasma densities and etch rates in a magnetically enhanced plasma etch reactor, and for an apparatus adapted to implement such a method. In particular, there is a need in the art for a magnetically enhanced plasma etch reactor which reduces or eliminates the plasma depletion effect observed at regions of the substrate disposed closest to the vertices formed by adjacent electromagnets. These and other needs are met by the devices and methodologies disclosed herein.